The pressing of metal powders for the production of porous metal bodies is known. To produce the desired porosity the so-called place-holder material dummy material can be added to the metal powder to enable the desired porosity to be stabilized. After pressing of the green body from the powder mixture, the place holder material is then removed from the green body so that the green body consists only of the remaining metal powder framework which has spaces within its framework structure. The green body has thus already the porous structure which is later to be found in the molded body. In the driving off of the place-holder material, one must be concerned to maintain the metal powder framework. By means of the subsequent sintering of the base body, a high porosity molded body can be obtained in which the powder particles are diffusion bonded together at their contact surfaces by sintering.
As the place-holder material or dummy material for the formation of porous metallic molded bodies, it is conventional to use relatively high melting organic components which by vaporization or evaporation or pyrolysis (cracking) and the solubilization of the resulting product by means of appropriate solvents can be removed from the green bodies. It is a problem with such materials that significant time is cost by the removal of place-holder materials and cracking products which can react with practically all of the metals used in powder metallurgical processes like titanium, aluminum, iron, chromium, nickel, etc. so that high concentrations of impurities remain. It is also a disadvantage where thermoplasts are used and are to be removed by heating the green body, that the expansion at the glass transition point has a detrimental effect on the requisite stability of the green body.
Alternatively, high melting inorganics, like alkali salts and low melting metals like magnesium, tin, lead, etc. are also used as place holders [dummy materials]. Such place holder materials are removed in vacuum, or under a protective gas at temperatures between about 600° C. to 1000° C. from green bodies at high energy cost and in a time-consuming manner. With such place-holder materials impurities will remain in the green body which may be detrimental especially in the case of molded bodies of reactive metal powders like titanium, aluminum, iron, chromium and nickel.
From DE 196 38 927 C2, a method of making highly porous metallic molded bodies is known in which initially metal powder and a place holder are mixed and then pressed to a green mass. In this operation both uniaxial as well as isostatic pressing can be used. The place holder or dummy is then thermally driven out and the green body then sintered. If the powder-dummy mixture is stabilized with a binder, it is in principle possible to produce even relatively complex component geometries by multiaxial pressing. The fabrication of the pressing dies for this purpose is however expensive and difficult. Especially for small series of pieces it is therefore advantageous to produce semifinished products or blanks with a universal geometry (for example cylinders or plates) and then by subsequent mechanical processing to impart the desired final contour to the product.
According to the present state of the art, the final shape is imparted to highly porous shaped bodies only after the sintering by conventional mechanical methods like for example turning, milling, boring or grinding. It is a disadvantage of these subsequent machining operations that the already sintered blank is connected with a local workpiece deformation. Through the plastic deformation there is usually a smearing of the pores. As a consequence the desired open porosity of the molded body is generally lost precisely in those surface regions at which it is desirable. This has a detrimental effect on the functional characteristics of the molded body. Furthermore, the workpiece, because of its porosity can only be clamped and machined with great care since it is not very stable under compression. The nonuniform surface of the porous molded body gives rise to a relatively high tool wear.